SUPERCONDUCTORS SEE THE LIGHT AT SHORTER WAVELENGTHS

January 28, 1999

A University of Rochester scientist and his Russian
colleagues from Moscow State Pedagogical University have
developed a superconducting device capable of detecting light at
wavelengths that were previously off-limits to the materials,
with remarkable speed and sensitivity.

The structure detects light in a portion of the infrared
spectrum that is important for telecommunications and infrared
astronomy, from 3 to 10 micrometers. The superconducting
material, niobium nitride, is capable of detecting just a single
photon, and it can recognize changes in light signals as fast as
25 billion times each second (25 gigahertz). Details of the
device, along with the ultra-fast measurements of its capability,
were published in the December 28 issue of Applied Physics
Letters.

"Detecting single photons is amazing, and ours is one of a
few detectors that can do so," says electrical engineer Roman
Sobolewski of the University. "But what really distinguishes our
device is its speed -- 25 gigahertz is very fast for an infrared
detector." Sobolewski says conventional infrared detectors are
typically either much less sensitive or slower.

In some ways the instrument, known as a hot-electron
photodetector (HEP), is "a very sensitive electron thermometer,"
Sobolewski says. When infrared light hits it, the temperature of
its electrons goes up. At an atomic level, when a photon hits the
niobium nitride, an electron absorbs it and becomes extremely
energetic or "hot." This rogue electron goes on to collide with
other electrons, which in turn run into still others, causing a
cascade, rather like a snowball rolling down a hillside and
gaining in size. The temperature of these excited electrons
quickly rises enough that the material itself temporarily loses
its ability to be a superconductor, or carry an electric current
with no resistance. The result is an electrical signal that
engineers can readily detect.

The type of light the detector captures is particularly
important in telecommunications. Signals sent from Earth to
satellites and back again travel in the range of 3 to 5 or 8 to
12 micrometers, in wavelengths that allow them to pass through
Earth's atmosphere unscathed. Another possible application down
the road: detectors for optical systems whose fibers would carry
such light pulses. In astronomy such wavelengths capture tales of
stellar birth and of the existence of planet-like objects outside
our solar system.

Engineers have long used superconducting materials in other
configurations to detect energy at longer wavelengths; this work
marks one of the first times a superconducting material has been
used to detect energy at shorter wavelengths, in the infrared.
Light at these energies is currently detected by other methods,
including semiconductors, which must be carefully grown and are
expensive to make.

The team's device is simply a single thin layer of niobium
nitride less than one-thousandth the thickness of a human hair
that works at temperatures below about 15 degrees Kelvin. After
absorbing a photon the material bounces back almost immediately,
returning to its superconducting state within 40 trillionths of a
second, or 40 picoseconds. The device works so fast because only
electrons are heated up; the material's temperature remains very
low. Such speed, combined with its small size and its ability to
detect infrared light, gives the material potential as one
component of a new type of computer known as a superconducting
computer. The University of Rochester is one of three academic
institutions in the country working on such technology.

The U.S. and Russian scientists involved in this project owe
their collaboration to the U.S. Office of Naval Research, which
sponsored the work in an effort to promote international
cooperation among scientists in the post-Cold War era. The films
were made and tested in Moscow, and the speed of the detector was
measured at the University, whose engineers have long been known
for their expertise in ultra-fast measurements.